Vacuum & Cooling

The opening of NOTCam in May 2011 was not planned and we had to wait for some spare parts that were ordered. Consequently, there was no time to do a proper baking of the cryostat to have it ready for the next scheduled run. We obtained a vacuum of around $2\times10^{-4}$ mbar, which is about an order of magnitude worse than the typical vacuum from 2008 and onwards. We believe the main reason for this is due to out-gassing of the installed components (the new flexible couplings).

A very precise anti-correlation has been observed between the cold-table temperature and the pressure. The pressure increases slightly when NOTCam is full of LN$_2$ and decreases when the LN$_2$ content is low. Since we got the new filling nozzle made of the super-insulating material ``torlon'', the thermal stresses on the filling tube have decreased substantially, and what we see now is likely related to thermal stresses on the vent tube when LN$_2$ is spilling out from a full tank. The plan is to exchange the fitting on the vent tube with torlon, as well.

The compressor oil filter of the PTR cooler was exchanged for the second time with a new refurbished unit from Iwatani, Kelvin International Corporation, after 63199 hours of total runtime. The cost of this was $4225. The previous exchange for maintenance was done at 28316 hours runtime. Recommended exchange is at every 15000 hours, but experience shows that (in our case) the oil filter lasts for at least 30000 hours.